my daughters birthday is incorrect and I need to change it

To change a birth date to any age below 13,* you must be a member of a Family Sharing group, and your family organizer must approve the change. After you change a birth date to any age below 13, you can't edit it.* Learn more about Family Sharing. 

1. Update the birth date.
2. Follow the onscreen steps to send an email request to your family organizer.
3. Your family organizer will get an email with instructions to approve the request. 

If she is indeed older then 13 maybe she can follow this steps instead

1. Go to Settings > [your name].
2. Tap Name, Phone Numbers, Email. You might need to enter your Apple ID password.
3. Tap Change Birthday or Birthday. 
4. Update your birth date, then tap Done. If you're changing a birth date for a child, learn what to do. 

See this link for further reference

Update the birth date associated with your Apple ID - Apple Support

The instructions for changing the date of birth for an Apple ID are in the support document Update the birth date associated with your Apple ID - Apple Support

Note that although your daughter is 15 she no longer has a child account. Different rules apply for accounts with dates of birth under 13.

You said she is 15 but didn't mention the age recorded in the Apple ID account. If the recorded date of birth reads as being under 13, and therefore the Apple ID is considered a child's Apple ID, observe Apple's note at the foot of the linked document:  If you need to change the birth date for a child, contact Apple Support.